Launcher3: Reduce app label text size

The default 13-point label size can barely fit any app names on modern
phone form factors without ellipsizing. Reducing the size to 12 points
allows us to fit most apps names, and upon visual inspection, it appears
to be the same size as that of Pixel Launcher.

Change-Id: I6ad63c14237f1f38c86ec02059b03896e5dd3a77